Regular expressions

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Regular expressions (or regexp) are used in programming to identify a template, a model, or elements that repeat in a predictable manner. They are a special sequence of characters that help you match or find strings or sets of strings, using a specialized syntax. Regular expressions reduce development time, because it takes only one line of code to check that the user input is a valid one for, let’s say a proper email address.

Since java 1.4 access to regular expressions is included in the JDK, via the java.util.regex package.

When matching a string to a pattern there are two ways on how this can be done:

  • By matching only part of the string
  • or

  • By matching the whole string

Matching part of the string
When trying to find parts inside a bigger string that match a given pattern you can use the java.util.regex.Matcher.find() function which starts searching the given string from its beginning to its end, identifying groups of characters that match the pattern.
Example:

This piece of code is trying to find groups of characters that match this format:
a whitespace character – \\s, followed by an uppercase letter – \\p{Upper}, followed by a digit – \\d and another whitespace character.
Suppose the input string to be matched (the line variable) has a value of:
“1. S1 2. W1 3. S3 some characters”,
the above code will find three groups of characters “S1”, “W1” and “S3” with the help of the group() function.

As long as the find() function returns true, meaning a match was found, further information can be obtained by using the start(),end(), or group() function.

Matching the whole string
The difference between finding only pieces of information and matching the whole string to a given pattern stands in a bit of a different pattern syntax:

Notice that the pattern syntax starts with a ‘^‘ character – this signifies the start of the line, and ends with a ‘$‘ character – this marks the end of the line. Without this syntax, the concept of matching an entire line of text, is not validated.

This pattern starts with a digit – \\d, then it is followed by a whitespace character – \\s, then it is followed by a group of minimum 1, maximum 9 word or non word characters – (\\W|w){1,9} and a whitespace character followed by any other characters – ‘.*’ until the end.

Regular expressions can be used to search, edit, or manipulate text and data.
For further reference on Java’s pattern syntax, see here

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